add an additional language fixture and update tests
fix required checkbox labels

Only the first label of a required form field should be prefixed with an asterisk.
make sure at least one language is available at frontend and session + cache gets cleared after delet

ensure that at least one language is available at frontend
- on edit via model validation
- on deletion via afterDelete

ensure "languages" cache gets cleared via afterDelete
ensure "Wasabi.content_language_id" session gets cleared if the language it points to has been deleted

Closes #1